A farmhouse sits next to a fracking site near Tioga County, Pa. / Robert B. Jackson

by Dan Vergano, USA TODAY

by Dan Vergano, USA TODAY

Drinking water wells near natural gas "fracking" sites were six times more likely to be contaminated than others, finds a new study of New York and Pennsylvania homes.

A nationwide boom tied to hydraulic fracturing, or fracking, has revolutionized the picture for U.S. energy production in the past decade. Natural gas production is up about 30% since 2005. Concerns about environmental effects on air and water from the wells, which shatter layers of shale deep underground to release gas or oil, have also risen. An Environmental Protection Agency review of well safety is underway and scheduled for release next year.

In a new study released Monday by the Proceedings of the National Academy of Sciences, a research team led by Robert Jackson of Duke University sampled 141 drinking water wells across northeastern Pennsylvania and southern New York. All of the fracking wells were in northeastern Pennsylvania. The results add to a 2011 study that first linked closeness to fracking wells to drinking water contamination with methane.

"it is looking like we are seeing a problem with well construction in some places and not others," Jackson says. Along with finding methane more likely to be contaminating drinking water wells within about 1,000 feet of fracking sites, the study found propane in 10 nearby wells. Ethane gas was 23 times more likely to be seen in homes similarly close to fracking sites. The ethane and propane are signatures of fracking, Jackson says. The gases probably escaped from leaks in the steel or concrete casing lining the uppermost part of the natural gas wells, called the wellbore.

On the plus side, there was no sign in the well's drinking water of industrial fluids used in the fracking process, such as diesel fuel. Fracking forces water and sand laced with these industrial fluids down deep and then sideways under high pressure to crack shale layers and liberate natural gas. There was no evidence of wide-scale leaks of methane far from the fracking sites beyond naturally occurring amounts seen in recent U.S. Geological Survey reports from the region.

"Some of the problematic wells are most likely due to some of the issues around casing and cementing problems" in northeastern Pennsylvania, says geoscientist Susan Brantley of the Earth and Environmental Systems Institute at Penn State. In one sense, she says, the study lessens wider concerns about fracking in the Northeast because it shows only 7% of water wells were contaminated, compared with 22% in an earlier study done by the same team.

On the other hand, Jackson says, the results do largely confirm that the methane seen in nearby wells is coming from fracking and not from natural leaks from more shallow layers of shale. The study found a slight increase in water well contamination near older fracking sites, which could either be a sign of leaks increasing as casings age or better seals and oversight of newer natural gas wells at work. The Pennsylvania Department of Environmental Protection issued 32% more citations for faulty fracking well casings from 2010 to 2011, a sign of increased regulation.

"Or it might be that we just don't know yet," Jackson says. He notes that his team released a similar survey of drinking water wells near fracking sites in Arkansas last month and found no signs of contamination at all. "Every fracking site is different and every place's geology is different. That may be what we are seeing here," he says. "It's clear that we need to focus on well integrity from these results."